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Last week I gave a very brief introduction to the fundamentals of electricity. I mainly focused on what an amp was and explained the dangers of even small currents. What I didn’t explain was how we get to amps from the other pieces of Ohm’s law: volts and resistance.

Have you ever plugged a flash drive or other USB device into your Apple USB keyboard and had your machine tell you there wasn’t enough power to run the device? It’s an annoying problem, but why does it happen? USB is primarily a data transmission bus, but it can also supply power. How much power? Well, the tiny wall adapter that charges your iPhone will output about 1A at 5VDC (5 watts). The iPad chargers are a bit beefier and output 2A at 5VDC (10 watts). Most USB ports on computers however are limited to 500mA. This is why plugging your iPad into your computer won’t necessarily charge it. It’s also why you get that warning about being unable to power the device. If you daisy-chain another device into a USB device like a keyboard, that single port has to supply power to both of those devices, and that may exceed the 500mA limit.

As I said last week though, between 0.1 and 0.2 can be lethal. How are people not constantly dying from USB-related electrocution? In order to have current, you need to have voltage. Voltage is the pressure that makes the charges move. Higher voltage equals higher pressure. Zero volts means zero pressure, and that means zero amps: I = V / R. 5VDC like from a USB port is not a lot of pressure at all. If Ohm’s law was I = V though, 5VDC would mean 5 amps! Yikes! But there’s one more piece: resistance. Every medium that electricity flows through has a resistance. Based on the equation, holding the voltage constant, a bigger resistance means a smaller current. We use copper wires because copper has a very low resistance. Therefore we can move the most charge with the least volts. But if you had enough voltage, you could make a circuit out of anything…even a human body, which is what happens during electrocution.

It’s difficult to measure the resistance of the human body. It depends on several factors including distance between the connections, and whether or not the skin is wet, or broken. Wet or broken skin will lower the resistance substantially. Common numbers given range from 100,000 Ohms on dry, normal skin, to just 1,000 Ohms on wet or broken skin. It’s all very complex and depends on many, many factors. However, these numbers alone are enough to show us why no one is dying from USB-related shocks. If we take the worst case scenario and say that we touched the positive and negative terminals of a powered USB connection on wet skin, how much current would flow? 5VDC / 1000 Ohms = 0.005 amps. You’d barely feel that. If it was done on dry skin, with a 100,000 Ohm resistance, you wouldn’t feel anything.

This is why high voltage is dangerous. The high resistance of your body is not enough to resist the higher pressure created by high voltage. The 120VAC from common household electrical outlets is orders of magnitude greater than the measly 5VDC coming from your computer. This is why you can get a pretty good shock if you touch live wires in a house. 120VAC / 1000 Ohms = 0.12 amps = death. Again though, the exact resistance provided by your body can vary widely. Whether or not the voltage is AC or DC affects the risk characteristics as well.

When you get into especially high voltages, like power transmission lines, microwaves, or other types of voltage step-up transformers, you simply don’t have enough resistance to protect you. The voltage pressure will overwhelm the resistance and create a deadly current. But even this isn’t the whole story. Did you know that your barbecue lighter lights the gas in its chamber using a spark gap created by a voltage potential of thousands of volts? Look out for next week’s article where I’ll explain some of the nuances of electrical systems and why no one is dying from barbecue-lighter-related electric shocks.

Pirate is getting better at learning where the “outside” is but we still have to carry him downstairs in the morning to go out or he will not make it all the way. He’s getting heavier now, too, so he had better pick it up soon! We’ve been puppy-proofing the house but he is a mischievous little guy.

It looks like some good weather for the weekend. I’ll have to see how far I want to ride to get some sweet corn or find one of those “hole-in-the-wall” restaurants for lunch. Grace has been feeding me from the garden which really makes this time of the year special, knowing exactly where your food came from!

Thank you for reading this issue of Kibbles & Bytes!

Your Kibbles & Bytes team,

Don, Emily, Hadley & Amy

Here’s a repeat of an article from Kibbles & Bytes #59! Hard to believe we are on issue #998. We will have to do something special for issue #1000 in a couple weeks. Anyone have any ideas?

Way back in 1998 Apple made some product intros….cue wavy screen, eerie music and the whirling time disk….

Apple’s new product announcements were exciting this week. I was able to get away to go down to Boston for the dealer briefing the day before the announcements and got some details on the new G3 PowerBook but even at that briefing they did not mention the new iMac. Steve Job’s announcement of the iMac reminded me a lot of the introduction of the Mac Plus. In fact if you take away all the power of the G3 and the cutting edge technology what you have is a Mac Plus on steroids. And one powerhouse of a Mac it is with 233MHz G3 processor and a built-in 15” display. This is the first Mac to come standard without a floppy drive and to utilize the new USB (universal serial bus) ports in place of ADB and serial ports. It is bundled with the new Apple USB keyboard and mouse. USB will be the standard for both Mac and PCs within a very short time. With Apple’s version of USB you get hot swappable serial ports that are automatically configured each time a device is connected (bet it’s Windows 2010 before wintel gets that one right!). At $1299 retail it will be a trend setting machine when it is released this fall. The all-in-one design has translucent plastics reminiscent of the eMate.

The iMac is squarely aimed at the consumer market and was key in no small part to bringing Intuit back into the Apple camp in terms of development of future versions of Quicken. When Apple discontinued the Performa line sales of the bundled Quicken program dropped dramatically and Intuit decided to stop development until they were shown the iMac. They then became one of the software packages to be bundled with the new machine. Apple’s consumer strategy assumes that end-users will not want to upgrade the processor on their internet machine as the processor is not upgradeable and there are no PCI slots. On the other hand just the same as with that old Mac Plus I am sure people will figure out a way to do things with this machine that Apple never envisioned.

The news of this machine saw Apple’s stock shoot up over 30 for the first time in a while and is already having an impact on the prices of other Macs. Along this line we have made some deep cuts in the pricing of the entire UMAX product line and you will see some price reductions in Apple’s refurbs as well.

The other announcement that almost got lost in the excitement over the iMac was the new G3 PowerBooks. These are some very interesting products. Without a doubt they are the fastest laptops available. PERIOD. There is no other portable computer that is as fast as these PowerBooks. That’s a pretty impressive statement. The new Apple advertising campaign “This new PowerBook eats the Pentium II for lunch” is hot! With the top of the line machine sporting a 14.1 inch active matrix display the line between a desktop and portable is also blurring a bit. The fastest of these PowerBook benchmarks out at around 1100 making it as fast as the fastest desktop G3s. Again hidden in the details are some interesting technological advances. These PowerBooks have S-Video input and support DVD drives (not the ones that will show movies!). They also have improved the case design and the keyboard. The lowest-end model features a SVGA (dual scan) color display and a G3 processor without backside cache. That model sells for around $2299 but still benches out near 500.

It is that time of year again for new Apple gear and you can stay informed right here in Kibbles & Bytes!